<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <style>
        :root{
            font-size: 5vw;
        }
        div{
            height: 50vw;
            width: 50vw;
            background-color: violet;
            border: 10vw solid skyblue;
            background: linear-gradient(to top left, green, blue);
        }
    </style>
</head>
<body>
    <div>
    </div>
</body>
</html>